This "highly polymorphic species" is widespread across eastern states, and its variation needs further study; typical glabra may be tetraploid (2n = 64) but more estimates are needed (Cr, FNA 3). Although often largely glabrous, buds and leaves can be pubescent. Leaflets can number 3-9 but are mostly 5 or sometimes 7. Nuts have smooth sutures or occasionally narrow wings. There may be local introgression with ovata (see notes under X ovalis), and there may be occasional hybridization with several other species; an exception appears to be laciniosa (FNA 3; W). Segregates have generally not been recognized within Ky. Typical glabra in this state has relatively small pyriform fruits with a contracted base that is sometimes stalk-like; they become blackish when mature. Trees with nuts that have more splitting sutures and narrow wings may be provisionally grouped with "ovalis"; see notes under that name. Also expected here is var. megacarpa (Sarg.) Sarg. (?= C. leiodermis Sarg.), which has larger non-pyriform fruits with thicker husk, and often 7 leaflets with a relatively large terminal one. Athough var. megacarpa is mostly known from lowlands of the southeastern Coastal Plain, it is reported north to s. Ill. (M, W), often growing close to laciniosa. The state champion of glabra (also close to the national champion) is in ALLE (KDF 2000): 137 cm dbh, 45 m tall, 22 m wide. Another large tree occurs in the Middleton Woods of HART.